Base stations in wireless communications systems provide wireless connectivity to user equipment within the geographic areas, or cells, associated with the base station. The wireless communications links between the base station and each of the user equipment typically include one or more downlink (or forward) channels for transmitting information from the base station to the user equipment and one or more uplink (or reverse) channels for transmitting information from the user equipment to the base station. Multiple-input-multiple-output (MIMO) techniques may be employed when the base station and, optionally, the user equipment include multiple antennas. For example, a base station that includes multiple antennas can transmit multiple independent and distinct signals to multiple user equipment concurrently on the same frequency band.
For example, consider a cellular system with M antennas at the base station and N antennas at the user equipment. In such communication systems, the radio channel between the base station and the user equipment can be described in terms of N×M links (sub-channels). Each link typically has a time-varying complex gain (i.e. amplitude and phase). If the radio channel is wide band (i.e. the signal rate is greater than the delay spread of the channel), the complex gain varies across the bandwidth of the transmitted signal. The overall state of the radio channel can therefore be described as a series of complex weights. This channel state information is measured by the user equipment and fed back to the base station in order to allow the base station to adapt characteristics of the signals transmitted to the user equipment to match in the most appropriate way to the prevailing channel state in order to improve, for example, throughput.
Although techniques exist to provide channel state information feedback, they each have their own shortcomings. Accordingly, it is desired to provide an improved technique for providing channel state information.